This invention relates to telecommunications systems which use digital modulation techniques to transport signals.
There is an increasing demand for supplying broadband telecommunications services such as high bit-rate data and video-on-demand services to subscribers. Traditionally the subscriber loop between a subscriber and their local telephone exchange has been a twisted-pair cable, with higher capacity trunks transporting signals between exchanges and connecting exchanges to service providers. When it is required to deliver broadband services it can be seen that the subscriber loop is the weak link in the delivery chain. Replacing twisted-pair cables by higher-capacity optical fibre or coaxial cables provides the required improvement in the subscriber loop capacity but at a prohibitive cost. However, it has been found that by using digital modulation techniques such as Discrete Multitone Modulation (DMT) together with digital signal processing techniques at the transmitter and receiver, high bit-rates can be successfully carried over existing twisted-pair subscriber loops.
Several digital transmission schemes are already in use. Asymmetric Digital Subscriber Line (ADSL) provides a high-capacity channel in the downstream (service provider to subscriber) direction and a lower capacity upstream control channel in addition to a conventional analogue voice channel. High bit-rate Digital Subscriber Line (HDSL) provides a symmetrical high-capacity link in both the upstream and downstream directions over one or two wire pairs. Both of these schemes use discrete multitone modulation.
Further research has shown that even higher bit-rates can be carried over the subscriber loop. Increased bit-rates allows higher-quality video services or an increased selection of services to be provided. Very high-speed Digital Subscriber Loop (VDSL) is a proposed scheme, also using DMT techniques, with an increased capacity of around 25 Mbit/s. VDSL occupies the spectral band up to 10 MHz on a subscriber loop.
An article entitled xe2x80x9cPerformance Evaluation of a Multichannel Transceiver System for ADSL and VHDSL Servicesxe2x80x9d by Peter Chow, Jerry Tu and John Cioffi, published in IEEE Journal on Selected Areas in Communications, Vol 9, No 6, August 1991, studies the performance of ADSL and VHDSL services including the effects of near-end crosstalk (NEXT) and far-end crosstalk (FEXT).
International Patent Application WO95/28773 (Amati) describes an ADSL compatible discrete multi-tone transmission scheme that can be used in the presence of crosstalk noise.
It has been proposed that VDSL use time-division multiplexing (TDM) techniques with duplexing achieved by transmitting in the upstream and downstream directions in separate time slots. VDSL generally operates over shorter distances than ADSL. VDSL signals may be coupled on to the subscriber loop at a street cabinet part-way between an exchange and a subscriber.
TDM schemes which use duplexing are not a good neighbour for other non-TDM schemes. This poses a problem of compatibility with existing and possible future schemes when duplexed TDM signals are carried over cables alongside wires carrying signals which are modulated according to other schemes.
The invention seeks to provide an improved digital transmission system.
According to one aspect of the present invention there is provided a method of operating a digital transmission system, the method comprising carrying on a first wire set a time-division duplexed multitone signal, and individually selecting the transmitted spectra of the upstream and downstream portions of the duplexed signal whereby to minimise cross-talk interference with further signals carried by other wire sets.
Preferably, in use, the transmitted spectra of the upstream and downstream portions of the duplexed signal differ in bandwidth.
The spectrum of the upstream portion of the duplexed signal may be selected so as to avoid the frequency bands used by downstream portions of the further signals carried by the other wire sets whereby to minimise near end cross-talk interference (NEXT) and the spectrum of the downstream portion of the duplexed signal may be selected so as to occupy substantially the full bandwidth of the wire set.
The upstream portion of the duplexed signal may occupy the bandwidth above substantially 1.1 MHz of the wire set to minimise NEXT with downstream ADSL signals carried by the other wire sets.
Preferably a multitone transmitter/receiver pair couples to each end of the first wire set, the step of individually selecting the transmitted spectra of the upstream and downstream portions of the duplexed signal comprising selectively using multitone sub-channels in the transmitter and the receiver of each pair.
Another aspect of the invention provides a digital transmission system comprising a plurality of wire sets, a first of the wire sets carrying a time-division duplexed multitone signal, the system being arranged to individually select the transmitted spectra of the upstream and downstream portions of the duplexed signal whereby to minimise cross-talk interference with further signals carried by other wire sets.
Preferably a multitone transmitter/receiver pair couples to each end of the first wire set, each pair having control means which individually controls selective use of multitone sub-channels in the transmitter and receiver.
Preferably each transmitter/receiver pair has a first filter at the output of the transmitter and a second filter at the input of the receiver. This provides additional shaping of the downstream and upstream spectra and minimises the effects of leakage into or from adjacent multitone sub-channels.
A further aspect of the invention provides a multitone transmitter/receiver pair for use at one end of a digital transmission system, which pair is arranged to transmit and receive a time-division duplexed multitone signal over a wire set, the pair having control means which individually controls selective use of multitone sub-channels in the transmitter and receiver whereby to individually select the transmitted spectra of the upstream and downstream portions of the duplexed signal to minimise cross-talk interference with further signals carried by other wire sets in the system.